Catch concurrent schema/table creation

[AlekSi]

Closes FerretDB/dance#28.

[codecov]

Codecov Report

Merging #283 (5e22df8) into main (3ef6cc2) will increase coverage by 0.12%.
The diff coverage is 80.76%.

❗ Current head 5e22df8 differs from pull request most recent head e5852b4. Consider uploading reports for the commit e5852b4 to get more accurate results

@@            Coverage Diff             @@
##             main     #283      +/-   ##
==========================================
+ Coverage   51.05%   51.18%   +0.12%     
==========================================
  Files         112      112              
  Lines        5347     5371      +24     
==========================================
+ Hits         2730     2749      +19     
- Misses       2205     2210       +5     
  Partials      412      412              

Impacted Files	Coverage Δ
internal/handlers/handler.go	50.34% <0.00%> (-1.05%)	⬇️
internal/pg/pool.go	76.36% <86.36%> (+0.12%)	⬆️
internal/handlers/msg_dbstats.go	75.00% <100.00%> (ø)
internal/util/testutil/testutil.go	82.66% <100.00%> (+3.50%)	⬆️

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